1. Field of the Invention
The present invention relates to a wavelength converter for converting the wavelength of an incident laser beam and generating a laser beam with a specific wavelength.
2. Description of the Related Art
Generally, a laser beam has characteristics that as the frequency is higher than that of an electric wave, it is excellent in information storage capacity, as the wavelength is equal and a laser beam is in phase, a laser beam is excellent in monochromaticity and directivity, has coherence which is not seen for a normal beam and further, as a laser beam can be extremely thinly focused, energy is focused on a minute area and high temperature and high voltage can be locally and instantaneously realized, and a laser beam is applied to many fields such as communication, information, instrumentation, processing technique and medical science.
For example, it is considered that in a stepper used for manufacturing a semiconductor device, an argon fluoride laser which oscillates by a pulse 193 nm in wavelength will be used in the future. However, for the argon fluoride laser, it is difficult to increase a repetition frequency so that it is a few kHz or more with reduced peak power. Efforts toward widening pulse length are made, however, currently it is not achieved.
Therefore, light intensity and light energy density on synthetic fused silica used in a stepper are required to be a fixed value or less so as to prevent the deterioration of transmissivity and the thermal gradient due to polarization and absorption, and to prevent compaction and aberration due to the occurrence of local optical path differences. Therefore, a demand for the performance of a laser such as the stabilization of pulse energy becomes extremely severe and the sensitivity of a photoresist is required to be enhanced. As a fixed number of pulses are required to be radiated to equalize optical density, it is difficult to enhance throughput.
As toxic gas required to be frequently replaced is used in an argon fluoride laser and the life of each high-priced unit constituting the laser is short, the maintenance cost is high.
In the meantime, a technique for generating an ultraviolet ray 194 nm in wavelength which is a continuous wave for a laser trap for confining a mercury ion is described on p. 4159 in Vol. 36 of Applied Optics written by D. J. Berkeland et al. and published in 1997. That is, this document describes a case that a sum frequency 194 nm in wavelength is generated by approximately 2 mW by radiating a second harmonic 257 nm in wavelength from an argon ion laser 515 nm in wavelength and an amplified laser beam 792 nm in wavelength from a semiconductor laser on a crystal of .beta.-barium borate (BBO) (.beta.-BaB.sub.2 O.sub.4). In this case, the BBO crystal is arranged in a position shared by both an external resonator which resonates with light 257 nm in wavelength and an external resonator which resonates with light 792 nm in wavelength and the optical paths of the two external resonators are spatially separated utilizing a difference in an angle of refraction by dispersion of the BBO crystal cut at Brewster's angle.
However, the above defects cannot be avoided by the above well-known technique when an argon fluoride laser is used, the apparatus is large-sized because an external resonator is used, reflectance and transmissivity are deteriorated because of adhesion of impurities to the surface of a mirror and others and output readily becomes unstable because of the misalignment of a mechanism. In addition, circuits for simultaneously locking external resonators with two wavelengths and actuators are required to be provided in only a step for generating a sum frequency and facilities for strictly matching the optical path length of the resonator are essential to lead light into the resonator.
If a higher harmonic from a solid state laser which can generate a high repetition frequency in place of an argon fluoride laser is used, the problems of the damage of synthetic fused silica and others and the cost are reduced. However, as an acquired wavelength is generally different from a wavelength generated by the argon fluoride laser, there is a defect that the above solid state laser is incompatible with the preceding argon fluoride laser.